1. Field of the Invention
The present invention relates to a process for producing a semiconductor silicon wafer. Particularly, the present invention relates to a semiconductor silicon wafer having not only a high gettering ability but also a denuded zone on the top side (surface) thereof, and to a process for producing it.
2. Related Background Art
Semiconductor silicon wafers are obtained by slicing a silicon ingot into wafers, and then various heat treatments are effected on the thus obtained semiconductor silicon wafers in the device processes.
A variety of gettering techniques are used to let metal contaminations, which were introduced into a semiconductor silicon wafer in the device processes, be captured in a region other than an element active region. The gettering techniques are classified into two large groups.
One of them is an intrinsic gettering (hereinafter referred to as "IG") process as shown in FIG. 12A, which utilizes precipitates of oxygen contained in the semiconductor silicon wafer in an amount of about 1.times.10.sup.18 (cm.sup.-3) as formed in the heat treatments. With heat treatments at a high temperature, at a low temperature and at a medium temperature in this order, outward diffusion of surface oxygen in semiconductor silicon wafer, nucleus formation for precipitates of oxygen and growth of oxygen precipitates occur, whereby the oxygen precipitates are formed only inside the semiconductor silicon wafer. An IG semiconductor silicon wafer is thus obtained with an element active region on the top side being a denuded zone. Although IG is generally known to have a higher gettering ability as compared with extrinsic gettering (hereinafter referred to as "EG") described below, it is extremely difficult to make the denuded zone on the top side completely free from faults.
The most generally used EG is a sand-blasting method, in which fine scars having a size of about 1 .mu.m or less are formed by bombarding the back side of the semiconductor silicon wafer with fine silicon particles. Then, the semiconductor silicon wafer thus obtained is oxidized to cause oxidation-induced stacking faults (OSF) or dislocation from the scars, so that the back side of the semiconductor silicon wafer becomes metal gettering sites (FIG. 12B). However, silicon (Si) could be peeled off in a scar portion so as to produce fine dust.
A recent method drawing attention as EG with high gettering ability is a process for producing a semiconductor silicon wafer having polycrystal silicon on the back side. This is a process in which polycrystal silicon is deposited by the vapor growth method (CVD) on the back side of a silicon wafer sliced off from a silicon ingot in the form of a wafer. According to this process, oxygen precipitates can be induced by the gettering effect in the polycrystal grain boundaries and by the heat treatments in the device processes, whereby both effects of EG and IG can be expected (FIG. 12C).
Incidentally, CMOS heat treatment simulation was conducted to measure the density of produced oxygen precipitates along the cross section of semiconductor silicon wafer, using two types of semiconductor silicon wafers made of silicon wafers of oxygen content [Oi]=13.times.10.sup.17 (cm.sup.-3) sliced off from a same ingot. One of them was produced by depositing polycrystal silicon on the back side of silicon wafer, while the other by EG in the sand-blasting method. The measurement results are shown in FIG. 13. As shown in FIG. 13, the semiconductor silicon wafer with back side polycrystal silicon has the density of oxygen precipitates about 2-order higher than that of the sand-blasting semiconductor silicon wafer, so that greater IG effect can be expected. On the other hand, the silicon wafer having back side polycrystal silicon shows, however, that the density of oxygen precipitates is higher in the vicinity of the top side. Such oxygen precipitates in the vicinity of the top side become recombination centers of carriers, which in turn cause degradation of device characteristics, or crystal defects.
Further, when the silicon wafer is subjected to a heat treatment at a high temperature in a reducing atmosphere for forming a denuded zone on the top side, employing the conventional extrinsic gettering method for forming a polycrystal silicon film on the back side of a silicon wafer, the back side polycrystal silicon film is recrystallized, whereby the thickness of the polycrystal silicon film might be considerably reduced so as to considerably lower the gettering ability.
Furthermore, various types of heat treatments at high temperature are performed in actual manufacturing steps of semiconductor devices, whereby the recrystallization of the polycrystal silicon film also occurs so as to lower the gettering ability.